Machines, including for example, on-highway trucks, wheel loaders, and excavators, utilize a variety of heat exchangers during operation. These heat exchangers may be used to increase, decrease, or maintain the temperature of oil, coolant, exhaust gas, air, and other fluids used in various machine operations.
In general, heat exchangers are devices that transfer thermal energy between two fluids without direct contact between the two fluids. A primary fluid is typically directed through a fluid passageway of the heat exchanger while a secondary cooling or heating fluid is brought into external contact with the fluid passageway. In this manner, thermal energy may be transferred between the primary and secondary fluids through the walls of the fluid passageway. The ability of the heat exchanger to transfer thermal energy from the primary fluid to the secondary fluid depends on, amongst other things, the heat transfer surface area of the fluid passageway (and associated structures) and the thermal properties of the heat exchanger materials.
Governments, regulatory agencies, and customers are continually urging machine manufacturers to increase fuel economy, meet lower emission regulations, and provide greater power densities. Due to these demands, the pressure and temperature differentials across heat exchangers are increasing. As a result, machine manufacturers must develop new materials and/or methods for increasing the ability of heat exchangers to transfer heat.
One method for improving the ability of a heat exchanger to transfer heat is described in U.S. Pat. No. 4,719,968 (the '968 patent), issued to Speros on Jan. 19, 1988. In particular, the '968 patent discloses a heat exchanger unit comprising a particulate heat exchanging mass or pack that consists of relatively small, mechanically immobilized particles. The immobilized particles are compressively retained in an enclosure in heat transfer relationship to each other and to a fluid directed therethrough. Preferred materials for the particles are crystalline carbon, copper and aluminum. The pack may be in cylindrical form and may be contained within metal conduits or, for solar radiation, within a transparent or translucent enclosure. The annular space between the conduits (one conduit being internal to the second conduit) may be packed with graphite particles having a thermal diffusivity which is of comparable magnitude to that of the encasing metal tube. Such an arrangement further improves the rate of heat transfer through a counter-flow fluid passing through the annular space. Also, the heat exchanger mass provides a significantly larger area of heat transfer contact between the particles and the fluid passing through the mass, as well as a multiplicity of minute flow channels to direct the fluid into intimate contact with adjacent heat transfer particles.
Although the heat exchanger of the '968 patent may provide a large area of heat transfer contact between the particles and the fluid passing through the mass, it may still be suboptimal. For example, using mechanical pressure to thermally couple the conduit to the pack may result in high thermal resistance. Also, the materials of the conduit and the pack may have different thermal properties (e.g., thermal conductivity and/or coefficient of thermal expansion). The difference in the thermal properties may cause the conduit to expand at a higher rate than the pack, resulting in loosening, cracking, and/or further increases in thermal resistance. Finally, for high flow rates, a large pressure may be required to immobilize the particles between the inner and outer conduits, thus creating undesirable stresses in the heat exchanger materials.
The disclosed heat exchanger is directed to overcoming one or more of the problems set forth above.